United States Patent
`
`[191
`
`Neukermans et al.
`
`[11] Patent Number:
`
`5,629,790
`
`[45]
`
`Date of Patent:
`
`May 13, 1997
`
`US005629'?90A
`
`R.N. Kleiman et al., “Single—crystal silicon high-Q torsional
`oscillators”, Rev.Sci.Instrum. 56(11), Nov.
`1985, pp.
`2038-2091.
`
`RA. Buser et al., "Very High Q—factor Resonators in
`Monocrystalline Silicon”, Sensors and Actuators, A2l—A23
`(1990). pp. 323-327.
`
`Breng, U. et al., “Electrostatic micromechanic actuators”,
`Journal of Micromechanics and Microengineering 2 (1992)
`256—26l. no month.
`
`Pfann, W. G. et al., “Semiconducting Stress Transducers
`Utilizing the Transverse and Shear Piezoresistance Eifects”,
`Journal ofzippiied Physics. vol. 32. No. 10, Oct. 1961, pp.
`2008-2016.
`
`Primary Exam¢'ner—la.mes Phan
`Attorney, Agent, or F:'rm—Donald E. Schreiber
`
`[57]
`
`ABSTRACT
`
`A frequency-locked torsional scanner of the type having a
`mitxronlachined mirror formed on a slnface of a silicon
`
`wafer section supported within a larger wafer section by a
`pair of opposed torsion bars. The principal vibrational
`frequency of the mirror is selected to be at least 20% higher
`than other modes of vibration. To prevent breakage, the
`torsion bars are hardened by conversion of at least a surface
`layer to silicon carbide or nitnide. A pair of scanners with
`orthogonal
`torsion bars may be mounted in a vacuum
`enclosure for two-dimensional scanning at difierent rates
`suitable for television display. In alternate embodiments, a
`detector and a scanner may be built on a plate on the same
`supported wafer section or two scanners may be indepen-
`dently supported or one scanner and one detector may be
`independently supported as two plates. The mirror may be
`driven eleclrostatically, magnetically, or by both methods.
`
`41 Claims, 11 Drawing Sheets
`
`[54] MICROMACHINED TORSIONAL SCANNER
`
`[76]
`
`Inventors: Armand P. Neukermans, 3510 Arbutus
`Ave, Palo Alto, Calif. 94303; Timothy
`G. Slater, 1226-25th Ave., San
`Francisco, Calif. 94122
`
`[2]] Appl. No.: 139,397
`
`[22] Filed:
`
`Oct. 13, 1993
`
`G021: was
`Int. Cl.“
`[51]
`[52] US. Cl. ........................ 3597193; 3597199; 3597201;
`3597202; 3597203; 3597214; 3597224; 2507234
`[53] Field ofsearcb ................................. 359:19s199,
`359201-203, 212-214, 223-226. 230,
`290-293; 2507230, 234; 310715, 36, 40 MM
`
`[56]
`
`References Cited
`
`Us. ecrnm DOCUMENTS
`
`3ll982 Petersen.
`8fl984 bkmkennmn "m"m""m""m".15H633
`311988 Gadhok.
`7Il990 Greenwood et al.
`5ll99l Cheiil’
`471993 Nelson
`
`
`
`25019201.]
`357726
`359!214
`
`4,317,611
`4A68282
`4,732,440
`4,942,766
`5,016,072
`5,202,785
`
`omen PUBLICATIONS
`
`Diem, B. et a1.. “S01 (SJMOX) As a Substrate for Surface
`Micro—machining of Single Crystalline Silicon Sensors and
`Actuators”, The 7th International Conference on Solidrstate
`Sensors and Actuators, (1993), pp. 233-236.
`K. Petersen. “Silicon Torsional Mirror”, Proceedings of the
`IEEE vol. 70, No. 5, May 1982. p. 61.
`B. Wagner et al., “Electromagnetic Microactuators with
`Multiple Degrees of Freedom“, International Conference on
`Solid—State Sensors and Actuators, Digest of Technical
`Papers, (1991), (IEEE Cat. No. 91CH2817—-5) pp. 614-617.
`V.P. Jaecklin et al., “Mechanical and Optical Properties of
`Surface Micromachined Torsional Mirrors in Silicon, Poly-
`silicon and Altrminum”, The 7th International Conference
`on Solid—State Sensors and Actuators (1993), pp. 958-961.
`
`
`
`0001
`0001
`
` Capella 2022
`Capella
`Cisco v. Capella
`Cisco V. Cape
`IPR2014-01276
`IPR2014—012
`
`

`
`U.S. Patent
`
`M
`
`5,629,790
`
`Wmo9I.13BPWu{
`
`R
`
`awIN
`Lw\I.._I
`
`IAW
`
`0002
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 2 of 11
`
`5,629,790
`
`12
`
`16
`
`FIG. 1a
`
`14
`
`14
`
`12
`
`16
`
`h:_h_ _ q__:,
`
`FIG. 1b
`
`FIG. 1c
`
`FIG.
`
`Id
`
`FIG. 1c
`
`0003
`0003
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 3 of 11
`
`5,629,790
`
`
`
`
`
`144
`
`142
`
`145
`
`I41
`
`131
`
`45
`
`143
`
`133
`
`135
`
`0004
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 4 of 11
`
`5,629,790
`
`0005
`0005
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 5 of 11
`
`5,629,790
`
`66
`
`64
`
`62
`
`FIG. 3a
`
`72
`
`70
`
`72
`
`68
`
`68
`
`FIG. 3b
`
`
`
`FIG. 3c
`
`0006
`0006
`
`

`
`US. Patent
`
`May 13, 1997
`
`Sheet 6 of 11
`
`5,629,790
`
`
`
`0007
`0007
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 7 of 11
`
`5,629,790
`
`0008
`
`

`
`130
`
`126
`
`124
`
`130
`
`128
`
`122 128
`
`FIG. 7
`
`0009
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 9 of 11
`
`5,629,790
`
`FIG. 10
`
`0010
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 10 of 11
`
`5,629,790
`
`173
`
`176
`
`174
`
`172
`
`134
`
`132 K)
`
`
`
`FIG. 9
`
`191
`
`197
`
`192
`
`194
`
`196
`
`FIG.
`
`11
`
`0011
`0011
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 11 of 11
`
`5,629,790
`
`209
`
`FIG. % 1221
`
`209
`
`2113
`
`225 EZZZZI I1 225
`czzzqzczzn
`203
`
`209
`
`211b
`
`207a
`
`207b
`
` « 221
`
`223
`
`-
`
`223
`
`FIG. 12b
`
`0012
`0012
`
`

`
`5,629,790
`
`1
`MICROMACHINED TORSIONAL SCANNER
`
`DESCRIPTION
`
`1. Technical Field
`
`The invention relates to optical beam scanners and. in
`particular, to micromachined vibratory scanners.
`2. Background Art
`Beam scanners are used in digital imaging. printing. bar
`code readers. optical reading and writing systems, surface
`inspection devices and various scientific and industrial
`implements. Such scanners deflect a beam of light, usually
`fi'om a fixed source. over an angle ranging from several
`degrees to tens of degrees. The beam sweeps back and forth
`at a frequency determined in part by the minor resonant
`frequency. A typical vibrational scanner of the prior art is
`described in U.S. Pat. No. 4,732,440 to J. Gadholr. The idea
`of making torsional scanners within a silicon body was
`proposed at an early date by K. Peterson, Proc. IEEE, vol
`70, no. 5, p. 61. May 1932. See also U.S. Pat. No. 4,317,611
`to K. Peterson.
`
`FIG. 1, depicting a scanner shown in FIG. 39 of Peterson,
`Proc. IEEE, supra, p. 61, includes a micrornachined tor-
`sional mirror 11, supported by torsion bars 13 and 15 within
`silicon body 17 (“micro scanner” hereafter). The aforemen-
`tioned article describes typical minor parameters, such as
`the modulus of silicon, the typical wafer thickness. the
`length of the torsion bar and the dimensions of the mirror.
`‘The width of the torsion bars is on the order of 500
`
`micrometers, while the length of the torsion bars is approxi-
`mately 0.2 centimeters- The mirror is approximately 0.22
`centimeters on a side. The out which isolates the minor from
`the silicon body and also defines the torsion bars is approxi-
`mately 0.02 centimeters in tbiclcness. Bach cut is made by
`anisotropically etching the silicon. The silicon body rests on
`glass substrate 21 which has vapor deposited electrodes 23
`and 25. A depression 27 is etched into the glass to receive
`silicon body 17 which rests on a linear support ridge 29. A
`high voltage is applied first to one electrode then the other
`in a continuing out-of-phase sequence from a drive circuit.
`The electric field generated by the electrodes tilts the minor
`first to one side and then the other. The restoring force of the
`torsion bars works against each defiection. The resonant
`fiequency of the mirror can be calculated with well known
`formnlas cited in the above-mentioned articles, although air
`damping creates an error in the resonance frequency. The
`substrate, electrodes and drive circuit are part of the micro
`scanner.
`
`Two dimensional rnicromnchined silicon flexure
`
`structures, used as gyroscopes, are known in the art. See
`U.S. Pat. No. 5,016,072 to P. Greilf. Such structures are
`similar to micro scanners in construction and vibratory
`characteristics.
`
`One of the problems encountered in the prior art is in
`restricting vibrations to a single desired torsional mode. An
`object of the invention was to devise a micro scanner which
`vibrates at a single desired mode of vibration and to be
`self-oscillating at its natural fundamental frequency.
`
`SUMMARY OF THE INVENTION
`
`The above object was achieved in a micro scanner having
`a primary vibrational mode, the torsional vibration mode,
`substantially lower in frequency from other modes by at
`least 20%. By providing the specified fiequency separation,
`the micro scanner will respond primarily to the desired
`mode. Mirror thickness,
`torsion bar length, mirror
`
`65
`
`0013
`0013
`
`5
`
`I5
`
`2
`dimensions, as well as drive characteristics can all define the
`vibrational mode spectra. Choices are made empirically. In
`contrast, while the prior art recognized the existence of other
`vibrational modes, no attention was paid to separation of the
`frequency of the lowest mode. We have discovered that by
`separating the torsional mode frequency from other vibra-
`tional modes, energy transfer into the principal vibrational
`mode, the torsional oscillation mode. is enhanced and other
`undesired modes are suppressed.
`It has also been discovered that other silicon structures
`can complement a micro scanner. For example, a second
`mirror which is arranged to vibrate out of phase with the first
`mirror can cancel torques injected into the support structure.
`Another example is a silicon detector which can be made to
`surround or be adjacent to the minor. An advantage of such
`a strucmre is that the minor and the detector can share
`common torsion bars so that the mirror and detector always
`point in the same direction and the mirrorreceives a constant
`collection angie. This solves the problem of correctly aiming
`a detector to receive a reflected beam. Another structure is
`a pair of plates, each supported by torsion bars within one
`silicon frame, with the plates driven in phase. Both plates
`may be mirrors or one plate may be a minor and another
`plate a photoderector. or both are combined mirror-detectors.
`Yet another structure is a two-dimensional scanner in which
`
`two sets of torsion bars are provided to two concentrically
`mounted frames supporting a single minor. One frame
`causes scanning at a first rate while the second frame causes
`scanning at a second rate. Such an approach would be useful
`for raster scanning because the horizontal scan rate is
`usually at a substantially higher frequency than the vertical
`scan rate.
`
`A micro machined container has been devised having a
`thin tough transparent window which can maintain vacuum
`conditions inside the container, but allow the beam to be
`reflected from the minor without substantial beam aberra-
`fions
`
`35
`
`45
`
`50
`
`55
`
`Micro scanners of the present invention may be driven
`electrostatically from the front or back sides of the mirror. or
`both. In addition, a magnet and galvanometer type drive may '
`be used above or in combination with an elecnostatic drive.
`An integrated torsion sensor is used for either stimulating
`self-resonance or as an angle sensing device for feedback
`control of minor position.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a perspective assembly view of a micro scanner
`of the prior art.
`FIGS. 1a—1e illustrate micro scanner mirror vibrational
`modes.
`
`FIG. 2 is a top view ofa micro scanner in accord with the
`present invention.
`FIGS. 2a, 2b, 2c and 2d are side plan views of alternative
`micro scanners in accord with the present invention.
`FIG. 3 is a top detail view of a portion of a torsion bar of
`a micro scanner in accord with the present invention.
`FIGS. 3a and 3b are side plan views of a method of
`making a micro scanner in accord with the present invention.
`FIG. 3c is a top plan view of the micro scanner shown in
`FIG. 31:.
`FIG. 4 is an electrical schematic of an elechical circuit for
`driving the elemodes of the micro scanner of FIG. 2.
`FIG. 4a is a waveform diagram for rectified cunent to the
`stripe electrodes in FIG. 4.
`FIG. 5a is a top plan view and FIG. 5b is a side plan view
`of a micro scanner of the present invention with a galva-
`nometer coil drive apparatus.
`
`

`
`3
`
`4
`
`5,629,790
`
`FIGS. 6 and '7 are top plan views of two dilferent
`embodiments of dual mirror scanners in accord with the
`present invention.
`FIG. 8 is a top view of a combined micro scanner and
`aligned photodetector in accord with the present invention.
`FIG. 9 is a plan view of the combined arrangement shown
`in FIG. 8 used in an optical scanning system.
`FIG. 10 is a top view of a dual in-phase micro scanner and
`photodetector combination in an optical scanning system
`FIG. 11 is a plan view of the combined arrangement
`shown in FIG. 10 used in an optical scanning system.
`FIGS. 12a and 12b show respective top and side views of
`a two-dimensional micro scanner in accord with the present
`invention.
`
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`With reference to FIGS. 1a—le, various vibrational modes
`of torsional scanners are shown. FIG. la depicts atop view
`of a desired or principal torsional mode of a micro scanner
`in accord with the present invention. This mode is desig-
`nated as mode 1 herein. The scanning mirror 12 has opposed
`axes 14 and 16 which twist in the direction shown by the
`arrows. In FIG. 1b, a vertical shaking mode is shown in a
`side view wherein the mirror 12 is moving up and down in
`the directions shown by the arrows. leaving a horizontal
`plane. This mode is designated as mode 2 herein.
`FIG. 1c shows mirror 12 in a vertical roclcing mode in
`which the mirror also leaves a horizontal support plane at the
`ends of the mirror, but not at the center. This is herein
`designated as mode 3. FIG. ld shows a lateral shaking mode
`where the mirror 12 moves first in one direction, then in an
`opposite direction within the support plane. This will be
`termed mode 4 herein. FIG. 1:: shows a lateral rocking mode
`in which the mirror 12 twists in one direction. then twists in
`an oppofite direction, within the horizontal support plane.
`This will be termed mode 5 herein. Modes 2-5 are
`undesired. but cannot be completely eliminated. Other
`modes. called plate modes. are possible but for most
`applications, the frequency of plate modes are much higher
`and would be removed if separation is achieved with respect
`to the modes which have been illustrated. More complex
`modes are also possible, again usually at higher frequencies.
`In the present invention. the frequency of modes 2-5 is
`separated from the frequency of mode 1 by a minimum
`frequency ratio. By maintaining an adequate separation,
`between the lower torsion mode and the next higher mode,
`less energy is transferred to these undesired modes. In the
`present invention. the frequencies of the various modes are
`shown in the following table for a typical configuration:
`
`TABLE 1
`
`
`Vibrational Frerpzency (Hz)
`
`Mode 1
`Mode 2
`Mod: 3
`Mode 4
`
`
`14,100
`
`1,500
`
`71,400
`
`3,200
`
`Fit‘-q
`Mid
`Freq
`18.500
`1.600
`930
`92
`Low
`Freq
`
`13,000
`
`'2l4,[I)C|
`
`5.500
`
`15,900
`
`adequately thick to withstand impact forces in the environ-
`ment in which the micro mirror is used. When driving a
`micnomachined mirror. certain forces described below are
`applied asymrnenically. For example, spaced apart elec-
`trodes can drive one—half of the mirror and then at a later
`time the other half. On the other hand. other forces. such as
`magnetic forces, cause a symmetric application of the driv-
`ing couple. In the symmetric application. the vibrational
`frequency of the torsional mode may be closer to the
`undesired modes. say within twenty percent. For the asym-
`metric application. the separation should be at least thirty
`percent.
`With reference to FIG. 2. a section 31 of a silicon wafer
`is shown. The wafer is etched to define a smaller wafer
`section 33 supported within the larger section 31 by means
`of torsion bars 35 and 37 which are integral to both sections.
`The surface of smaller section 33 is polished in the manner
`of commercial silicon wafers so that it has a shiny. reflective
`surface. Mounted either below or above the wafer and
`slightly spaced therefrom are the electrodes 41 and 43,
`indicated by dashed lines. These electrodes will be alter-
`nately charged by voltages which cause electric fields which
`attract the smaller section 33. hereafter referred to as mirror
`
`33. which is electrically grounded through the torsion bars
`and to the surrounding larger section. Note that there is no
`fulcrum or backplane support as in FIG. 1. Only the torsion
`bars provide support Apart from this. the overall design of
`the scanner mirror of the present invention. up to this point,
`may be in accord with the prior art. However, the mass of the
`mirror and the dimensions of the torsion bars and perhaps
`other variables are selected so that the torsional mode is well
`separated. Also, the electronic circuitry associated with one
`of the torsion bars. described below. as well as torsion bar
`support radii are new.
`In FIG. 2n, the electrodes 142 and 144, corresponding to
`electrodes 41 and 43 in FIG. 2. are shown on an insulative
`substrate 45. The larger wafer section has opposite sides 131
`and 133 which are disposed on the glass substrate 45 and
`have a rectangular shape similar to the section 31 of FIG. 2.
`The mirror 135 is supported by torsion bars from the larger
`silicon section in a position spaced above the electrodes 142
`and 144. Above the sides 131 and 133 at the larger silicon
`frame is a portion of a second wafer having opposed edges
`141 and 143. Optionally, the edges 141 and 143 support a
`vapor deposited very thin membrane window 145 (or any
`transparent window) if a sealed container is desired.
`The entire structure is fabricated using semiconductor
`processing techniques. Atop the dielectric substrate 45, the
`electrodes 142 and 144 are vapor deposited metal stripes
`which are patterned on the silicon dioxide coating on the
`substrate 45 using standard photolithographic techniques.
`The silicon section having sides 131 and 133 and the integral
`mirror 135 are separately fabricated by anisotropically etch-
`ing a silicon wafer. Only opposed torsion bars support mirror
`135. The micromachined silicon housing described above is
`preferred. but not necessary. A conventional box with a
`transparent top could also be used. When a membrane
`window is used, the window is made sufliciently tough so
`that transparent electrodes may be deposited directly on the
`membrane. With reference to FIG. 2b, electrodes 142 and
`144 are very thin indium tin oxide stripes deposited on
`window 145. The stripes may be only a few molecular layers
`in diickness because very little current is conducted by the
`electrodes.
`
`The thickness of the mirror 12, 33 or 135 may be equal to
`the thickness of the wafer, or less. For high fiequencies of
`optnation. the mirror thickness is typically a fraction of the
`
`10
`
`15
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`In general the separations described above for the tor-
`sional mode at any given frequency range are achieved by
`designing the torsion bars as thin and narrow as possible, yet
`
`0014
`0014
`
`

`
`5
`
`6
`
`5,629,790
`
`wafer thickness. Mirror thickness may range from less than
`one micron to tens of microns. The preferred method of
`manufacture, involves use of a Sirnox wafer, or similar
`wafers, e.g. silicon on insulator substrates, where the mirror
`thickness is determined by an epitaxial layer. Single crystal
`silicon is preferred both for the mirror and the torsion bars
`because of its superior strength and fatigue characteristics,
`as compared to metals or polysilicon. For low frequencies of
`scanner operation, typically below 100 Hz, if the 1-nirror’s
`thickness equals only that of the epitaxial layer, then the
`length of the torsion bars makes them too fragile to with-
`stand liquid processing or shock within their working envi-
`ronments. The full thickness of the wafer's epitaxial layer
`should be used to form the torsion bars in this situation. The
`torsion bars would now be much broader and shorter. but
`their thickness would still be set by the epitaxial laym’s
`thickness. However.
`the mirror would be much thicker
`equaling the total wafer thickness depicted in FIG. 3a. The
`wafer about the miJIor’s mass around the center can be
`
`mostly etched away producing a box frame structure such as
`that illustrated for the frame 207 depicted in FIGS. 12a and
`126. ‘This afiects the resonance frequency very little, as well
`as the moment of inertia, but reduces the mass of the mirror
`and hence the forces on the torsion bars. Construction of the
`thicker section is explained below with reference to FIG. 30.
`Once completed, the larger structure has a light transmis-
`sive window mounted above the scanning mirror. This is
`done by taking a second silicon wafer and vapor depositing
`a layer of silicon nitride, silicon carbide or boron nitride over
`the wafer and then etching away the supporting wafer down
`to the thin vapor deposited film. A thin layer of Si could also
`be used The edges 141 and 143 are sides of a second wafer
`structln'e joined to opposing edges I31 and 133 of the larger
`section of a first wafer structure. The two congruent wafer
`sections are joined by a variety of processes such as anodic
`bonding, silicon to silicon bonding, solder glasses, etc. all
`done in a vacuum environment. This creates vacuum con-
`ditions inside of a closed container. The method of manu-
`facturing the thin window 145 is described in U.S. Pat. No.
`4,468,282 to A. Neukermans. The patent describes thin films
`having a thickness in the range of a few microns. The area
`of the window for a micro scanner would be about 3 mrnX3
`mm. The advantage of such thin films is that optical aber-
`rations are eliminated. The film which is selected should be
`substantially transmissive of light, with little absorption so
`that the film will not be damaged by an incident laser beam.
`By providing a vacuum container for mirror 135, damping
`due to air is eliminated and the minor will oscillate to
`frequencies ranging up to several tens of thousand hertz. It
`should be noted that a vacuum enclosure is not necessary,
`but greatly helps in reducing the voltage needed for elec-
`trostatic drive, as well as for magnetic drive. Because the
`micromachined mirrors are difficult to clean, a dust cover is
`preferable. The windows, in a non-vacuum environment,
`serve as a dust cover. Large electrostatic voltages attract
`particles to the srnface of the mirror and so the enclosure
`serves several purposes.
`The mirror construction of FIG. 2c is similar to the
`construction of FIG. 2a. In this case, the insulative substrate
`245 supports the larger wafer section having side walls 231
`and 233. The smaller wafer section 235 is supported by
`torsion bars within a frame defined by the larger wafer
`section, as in FIG. 2. A second wafer 240 has a vapor
`deposited thin film window 245 thereover which is similar
`in materials and construction to the thin film window 145 in
`FIG. 2a. The second wafer 240 has side walls 241 and 243
`and a bottom wall 247 with an etched opening 249 below the
`
`10
`
`30
`
`35
`
`45
`
`50
`
`55
`
`film 245. The bottom wall portion 24'? overhangs mirror 235
`and has electrode stripes 242 and 244 on the underside of the
`bottom wall 24'! facing mirror 235. Just as previously
`mentioned. the electrode stripes 242 and 244 are conductive
`material that is vapor deposited once the bottom wall 24?
`has been formed by etching the second wafer and an oxide
`coating applied. The electrode stripes 242 and 244 perform
`the same function as previously, deflecting mirror 235 by
`electrostatic force generated by alternate high voltages
`applied to the electrodes. Because of the overhang of bottom
`wall 247 over the mirror 235. the opening 249 will not be as
`large as the opening provided in FIG. 2a where the minor
`drive force is from the rear of the mirror. It is possible for
`auxiliary stripes. not shown, to be placed below the mirror
`in FIGS. 2:: and 2!; so that electrodes are both above and
`
`below the mirror. Drive forces are synchronized between top
`and bottom electrodes so that diagonally spaced electrodes
`are both pulling. This symmetric pull relationship between
`electrodes above and below the mirror will strengthen the
`vibratory force applied to the mirror and will assist in
`principal mode selection because of the symmetry.
`In FIG. 2a‘ mirror 135 is made reflective on both sides.
`
`The thin window 145' has a central opening 150 which
`admits a beam 152 directed toward the center of vibration of
`the mirror. A similar beam 154 is directed to the back surface
`of the mirror. In this manner, both front and back surfaces of
`the mirror can deflect different beams.
`
`In FIG. 3, a detail of torsion bar 37, suspending mirror 33
`from the larger section 31 of a silicon wafer. The torsion bar
`37 may be seen to have rounded corners 32, 34 formed by
`an anisotropic silicon etch. The rounding of corners removes
`stress concetltrations. The radius of rounding should be at
`least equal to the thickness of the torsion and preferably it
`should be near the width of the torsion bar.
`
`In fabricating torsional scanners in accord with the
`present invention, commercially available Sirnox wafers are
`preferred as substrates for construction of the torsional
`scanners. With such wafers, the silicon left standing after
`etching, is single crystal and stress free. The silicon does not
`curl, which is extremely important for mirror applications.
`Three well-controlled thicknesses of the mirror plate and
`torsion bars are obtained, giving well—coi1trolled results over
`the entire wafer. Simox wafers have a built-in etch stop
`which greatly eases fabrication of mirror and hinges. Thick
`uniform mirror plates and torsion bars are made in this way,
`with thicknesses up to 100 microns. Germanium-
`compensated boron-doped silicon, and electrolytic etching
`of epitaxial layers can also be used. In FIG. 3a, a Simox
`wafer is illustrated. The top epitaxial layer 66 is a few to 50
`microns thick. the silicon dioxide layer 64 is about 2,000
`Athiclr and the base layer 62 is typically a 500 micron layer
`of single crystal silicon. Acavity is etched fiom the backside
`of the wafer, using standard and anisotropic etchants, such as
`EDP. This etch is automatically stopped at the oxide layer
`64. Subsequently, the epitaxial layer 66 of the wafer is
`patterned to define a mirror 70 and torsion bars 74. The
`oxide layer 64 is removed in the desired places and the
`exposed patterned silicon is etched in a reactive ion etch
`reactaor, using, for example, chlorine as an etchant. This now
`delineates in the epitaxial layer 66 the mirror '70 and torsion
`bars 74 and produces straight walled torsion bars 74. An
`anisotropic etchant could also be used, producing a trap-
`ezoidal cross-section for the .torsion bars 74. After removal
`
`65
`
`of all oxides, the mirror is free standing and can be coated
`with thin layers of metal or dlelecnic to enhance reflectivity.
`Note that in FIG. 3b, the cuts 72 in the epitaxial layer 66 are
`made concurrently with definition of the mirror 70 and the
`
`0015
`0015
`
`

`
`5,629,790
`
`7
`
`torsion bars 74 which support the mirror within the support-
`ing silicon wafer portion 68.
`To form the torsion bars. the front side of the wafer is
`patterned. The front mask is aligned to the back side of the
`wafer. with a two—sided aligner. Oxide layer 64 is removed
`after patterning and the exposed silicon is etched in a
`reactive ion etch reactor. using chlorine as an etchant. This
`procedure gives rise to a straight walled torsion bar 74.
`Alternatively. the cuts '72 and the torsion bars 74 may be
`etched using anisotropic etchants such as KOH or ED. or
`isotropic etchants such as HFIHNO3.-'1-I20. Oxide is again
`removed. leaving free standing torsion bars supporting the
`mirror.
`
`An alternate etch stop technique is the well-known elec-
`trochemical etch stop. In this method, an n—type layer is
`opitaxially grown on a p-type substrate. By applying a
`voltage to die n-layer during the etch. it is possible to etch
`the p-type substrate without etching the n-layer (see ref.).
`This method can be used to make n—type membranes of
`precisely determined thickness, which can then be patterned
`and etched to form mirrors.
`
`As an alternative to an etch stop layer. a plain silicon
`substrate can be time etched to form membranes of the
`desired thickness. which can then be patterned and etched to
`form mirrors.
`
`In achieving maximum deflection. breakage of the torsion
`bars is a risk. However. since cracks usually originate and
`propagate from the surface. the surface can be hardened by
`conversion of the surface into silicon carbide or silicon
`nitride. This is done by exposing the stnface of the bars to
`hydrocarbons or ammonia for nitridation at 900° C. This
`causes the top several thousand angstroms of silicon to be
`converted into silicon carbide or silicon nitride. See also the
`method of J. Graul and E. Wagner. Applied Physics Letters.
`21. No. 2. p. 6? (1972) relating to conversion of monocrys-
`talline silicon to polycrystalline [5-silicon carbide using
`methane. The scanner mirror must be protected with oxide
`during the silicon hardening process. The tensile strength of
`silicon carbide is approximately three times that of silicon.
`Referring again to FIG. 3. before the etching of the mirror
`and torsion bars. a first pair of contact pads 36 and 38 are
`aligned along the axis of the torsion bar. A second pair of
`contact pads 46 and 48 are transverse to the first pair. Each
`of the contact pads has a respective wire lead 44 deposited
`on the torsion bar leading outwardly to the larger section of
`wafer material and to electrical connection with the circuitry
`described herein. Contact pads 36 and 38 are provided for
`the purpose of establishing a ctn-rent flow generally coaxial
`with the torsion bar axis and between the contact pads 46
`and 48. Actually. only one pad 36 could be used as a current
`injection point and the two nearest pads used as current
`sinks. Torsion in the bar then causes the ratio of the currents
`to change. Upon twisting of the torsion bar. a voltage is
`generated between pads 46 and 48. The mutually orthogonal
`contact pads 36. 38 and 46. 48 may he used in two ways. In
`one mode, the torsion sensor is used for self-oscillation of
`the resonant structure. The generated voltage is used in a
`positive feedback scheme. by reinforcing this signal with
`force members which apply electrostatic or electromagnetic
`forces to the mirror, making the mirror resonate at its
`principal torsional frequency. In a second mode, the trans-
`verse voltage generated by the twisting minor is a
`transducer. measuring angular deflection of the mirror. and
`so the signal may serve to indicate the angle at which a beam
`is being deflected.
`In the situation where a DC voltage is applied across pads
`36 and 38. slight movements of the torsion bar are converted
`
`10
`
`15
`
`20
`
`30
`
`35
`
`55
`
`65
`
`8
`to vibrations at the resonant frequency of the torsion bar. In
`this sense. the electrical sensor is a feedback mechanism
`which helps the mirror attain resonance at the principal
`vibrational fiequency. If an AC current is applied to the
`sensor. say 20 kHz.
`then the torsion signal becomes a
`modulation envelope of the imposed frequency. The benefit
`of the modulation envelope is that it is easier to detect and
`thus monitor vibrational modes. beam position or the like in
`the presence of large drive signals. The torsion bar is
`preferably aligned in the <l10> direction for n-type silicon
`or the (100) direction for p-type silicon. These orientations
`are not only optimal for shear stress. but these arrangements
`are nearly insensitive to uniaxial strains in these orienta-
`tions.
`
`In FIGS. 4 and 4.5:. an electrical circuit is shown which
`provides the drive signal for the snipe electrodes 41 and 43
`in FIG. 2. A sinusoidal or square wave low voltage input
`signal is applied to transformer primary winding 51. The
`secondary winding 53 of a ferrite core transformer steps up
`the input voltage to a higher level at a 50 to 1 turns ratio. A
`commercially available part such as Phillips 3622 PL00-
`3E2A will suifice. The secondary winding 53 has a grounded
`side 55 and a hot side 57 which is rectified by one or more
`diodes 61 in a first leg and one or more reversed biased
`diodes 63 in a second leg. The diode string 61 of the first leg
`provide a rectified half wave 71 to electrode 43. The
`reversed biased diode string 63 provides a reclined half
`wave '73 to electrode 41 at peak voltages as high as 1000
`volts. High value bleed-ofl’ resistors 75 and 77. about 10M
`ohms. are used to discharge the plates. These voltages which
`are seen in FIG. 4a to alternate from positive to negative
`corresponding to one electrode then the other, pulling from
`one side then the other, causing mirror reciprocation.
`With reference to FIGS. So and 5b a torsional scanner 8]
`is shown having a central mirror 82, torsion bars 84, contact
`pads 83 to be used as a position sensor in accord with the
`description given in relation to FIG. 3. and a circumferential
`loop coil 85. The coil 85 is a conductive loop which may be
`formed by patterning or vapor depositing conductive mate-
`rial onto the silicon mirror 82 about the periphery thereof.
`The object is to establish a magnetic field within the coil
`perpendicular to the mirror. The coil or loop is insulated with
`silicon dioxide or another insulator. The conductive cross-
`
`over of conductor 87 is accomplished by well known
`layering techniques. Alternatively. conductor 37 can be
`terminated prior to the crossover location and. if the under-
`lying silic